J. H. Collins — Cornish Serpentinons Rocks, 221 



observed by myself. The base is green and transparent ; it includes 

 many minute acicular or fibrous crystals of hornblende or asbestos 

 crossing each other in all directions, some in bundles with parallel 

 axes, as if from the partial destruction of a cleavable crystal. In 

 some instances the hornblendic bundles are replaced by a greenish 

 mineral which is possibly chlorite, and there is always present more 

 or less crystalline magnetite. Mr. Phillips says, " It is worthy of 

 notice that while the slates of Botallack ai'e highly magnesian, the 

 sea-water which percolates through them into the workings of the 

 mine has lost three- fourths of its magnesium. Similar effects 

 appear to be produced at Huel Seton, where the amount of magnesia 

 in the rock bounding the great cross-course, which is traversed by 

 the modified sea- water constituting the well-known ' lithia spring,' 

 is tMdce as large as it is in the normal killas of the locality. The 

 magnesium of the sea-water has in this case almost entirely dis- 

 appeared." ^ 



Terras, St. Stephens. — This is a band of dark and fine-grained 

 rock used locally for roadstone, which is not marked on the 1-inch 

 Geological Map. It is very tough, and often platy in character, 

 owing to an incipient pseudo-cleavage which I take to be an indica- 

 tion of an original bedding now almost obliterated. It has all the 

 appearance of a compact hornblende schist. Thin veins of an 

 asbestiform mineral are often seen in the joints, and occasionally the 

 fibres of this mineral are so interlaced as to result in a kind of 

 " rock-leather " or " rock-felt " sometimes as much as half an inch in 

 thickness. Some of the joints, however, are lined with thin films of 

 serpentine, and in such cases the rock itself exhibits serpentinous 

 change to a depth of an inch or more, so as to be almost indistinguish- 

 able from the Botallack rock above referred to. In thin sections, 

 and under a moderate magnifying power, it is seen to be made up of 

 a transparent greenish serpentinous base full of matted crystals of 

 hornblende. The original laminar character of the rock is indicated 

 by many ci'ystals of magnetite and minute patches of viridite. 

 Occasionally, too, fibres of asbestos are visible, and, rarely, minute 

 prisms of apatite. 



I made a partial analysis of this rock in 1872, with results as 

 follows : — 



Loss on ignition "92 



Silica 45-20 



j\himina 19-80 



Magnetite 5-01 



Ferrous oxide 7-35 



Lime 8-00 



Magnesia 6-20 



Not determined 7-52 



100-00 

 A very similar rock, probably an extension of the same bed, was 

 opened up some years since at Terras Mine, and described by Mr. 

 J. A. Phillips as a greenstone (Q.J.G.S. vol. xxxii. p. 175). 



^ Zoc. cit. See also " On tbe Composition and Origin of the "Waters of a Salt- 

 spring' in Huel Seton Mine," PMl. Mag. July, 1873. 



